Fluid swivels are commonly used in offshore installations to transfer fluids such as gas and oil between an underwater pipeline or well and a tanker. A typical fluid swivel of this type includes a ring-shaped outer wall that rotates about a relatively fixed inner wall, the walls forming an annular chamber between them through which fluid passes. There are gaps between the walls leading in opposite directions from the chamber, and these gaps or gap passages must be sealed.
Prior art swivels have generally used radial seals to seal the gap between the inner and outer walls, each seal pressing radially inwardly against the inner wall and radially outwardly against the outer wall. The life of the seal depends upon changes in the thickness of the extrusion gap portion which lies on a side of the seal furthest from the annular chamber. The thickness of this extrusion gap portion greatly increases when high pressure fluid is applied, and the increased thickness decreases the lifetime of the seal. The use of face seals, or axial seals, which press in opposite axial directions against seal surfaces on the inner and outer walls, and with the extrusion gap portion extending radially, can greatly increase the lifetime of the seals, because the thickness of the extrusion gap portion does not increase greatly when high pressure fluid is applied. This is especially useful for fluid swivels that may operate at high pressures such as 2000 psi, and which have a large diameter such as an annular chamber diameter of at least about 4 feet.
While the use of axial or face seals reduces changes in the thickness of the extrusion gap portion, it can lead to problems in precisely controlling the thickness of the extrusion gap portion. The inner and outer walls of the swivel are formed by several parts that are stacked on one another. The desired thickness of each extrusion gap portion may be perhaps 10 mils (one mil equals one thousandth inch), and care must be taken that the sum of the tolerances of the stacked parts does not greatly decrease the gap thickness (which can lead to the walls rubbing on each other) or result in an excessively large gap thickness, (leading to short seal life). It is possible to hold the various parts to extremely small tolerances, but this results in much greater expense in manufacture. A fluid swivel with face seals along the gap passages, which enabled the maintenance of close tolerances in the extrusion gap passages, while enabling the fluid swivel to be constructed at moderate cost, would be of considerable value.